Real-world examples and case studies

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Introduction

Improper grounding practices may lead to problems in installations, sometimes merely annoying, but often with serious consequences to equipment. Practicing engineers face such problems often. A careful look at the installation often reveals the problems to a trained engineer and rectification thereafter is usually quite a simple matter. We present below some case studies based on problems encountered in actual installations and how they were solved. Some of the participants might have come across such problems and we welcome them to share their experience with the group.

Case study 1

Effect of utility-induced surges:

Problem---A steel mill with variable speed drives (VSDs) had problems of frequent tripping of the VSDs with the indication 'over-voltage in AC line'. Each tripping caused severe production disruption and resulted in considerable monetary loss due to lost production.

Steady-state measurements by true RMS voltmeter showed that voltage was normal and within the specified operating range of the VDS. A power line monitor was then used in the distribution board feeding the VSDs and the incoming power feeder to the mill. At both locations, the monitors showed transient over-voltages of damped oscillatory type waveform with an initial amplitude of over 2.0 pu and a ringing frequency of about 700 Hz.

The timing of disturbances coincided with the closing of capacitor banks in the utility substation feeding the steel mill (refer FIG. 1a).

Analysis---It was confirmed by the VSD manufacturer that the VSDs were provided with over voltage protection set to operate at 1.6 pu voltage for disturbances exceeding 40 µs. Since the switching transients were above this protection threshold, the VSDs tripped. It may be noted that switching on a bank of capacitors results in high charging current inrush. When this current passes through the line's inductance L, a momentary voltage surge occurs. Further interaction of the capacitor C with inductance L results in an oscillatory flow of current, which is damped by the resistance R in the system. The oscillatory disturbance superimposed over the normal power-frequency voltage wave caused the over-voltage protection to operate.

FIG. 1a Distribution arrangement

Solution---The solution lies in reducing the transient peak to a value that is below the over-voltage protection threshold. This was achieved in this case by installing a surge protection device (SPD) in each VSD. The SPD clamped the transient to a peak value of 1.5 pu thus avoiding the operation of over-voltage protection. Another possible solution would have been to install an inductor L1 in the switching circuit of the capacitor for a few seconds and then shunt it by switch S. Since the voltage seen by the incoming feeder to the mill would be the combination across C and L1, the transient will have a smaller amplitude. This solution will however call for cooperation from the utility as it involves additional equipment to be installed by them (refer FIG. 1b).

Case study 2

Effect of neutral breakage:

Problem---In an office building with several offices, a constant voltage transformer (CVT) feeding a facsimile machine got overheated one night and started emitting smoke. On getting the fire-alarm signal, the security guard switched off supply to the machine. The following day, the engineer who was investigating this problem noticed that several fluorescent lamps going randomly on and off with an abnormal humming sound from the lamp chokes. This was happening all over the office intermittently.

FIG. 1b After additions

Analysis---The office building was fed by a 500-kVA transformer, which fed a distribution center through a cable with three full capacity line conductors and a half capacity neutral conductor. It was found that in the incoming cable termination to the LV distribution center, the neutral lead was red hot and arcing intermittently when the loads were on. Many offices in the building were installing computer systems fed by UPS equipment in the previous 1 year. The non-linear nature of these loads had caused much higher neutral currents to flow compared to a balanced three-phase load, which the cable with reduced neutral was designed to carry. This had caused the neutral termination to overheat and deteriorate causing intermittent loss of neutral continuity (refer FIG. 2).

When the neutral connection was absent, the voltage across each phase group was decided by the load distribution between the three phases and most loads being of single phase type loads were not perfectly balanced. Refer to the impedance and phasor diagrams in FIG. 2.

The heavily loaded phase was thus having a smaller voltage and as the loads were being switched on in the morning hours, it had caused random voltage variation in different phases. This in turn resulted in flickering of the fluorescent loads and humming of chokes in the phases having higher voltage (lightly loaded phases). The CVT had burnt out due to prolonged over voltage.

FIG. 2 Distribution arrangement, impedance and phasor diagrams

Rectification---As an immediate step, the termination was repaired and the cable put back in service. As a preventive measure, the cable was replaced by one having a neutral conductor of the same size as the phase conductors. The system has been operating now for about 5 years without any failures.

Case study 3

Telephone equipment failures:

Problem---Frequent failure of telephone equipment causing lengthy service interruptions was occurring with a particular customer and the failures always coincided with lightning activity. The ground continuity of the surge protection device was checked by a continuity tester and was found to have no discontinuity.

Analysis---A physical check was made to ensure that grounding conductors and the ground electrode were not in anyway defective. It was then seen that a length of about 10 m of grounding wire had been looped in the form of a coil before being connected to the ground rod. The resistance was low enough for the continuity checker to show normal value. However, the discharge of surge currents through the SPD was causing an excessive voltage to develop across the inductance of the looped conductor rendering the SPD ineffective and resulting in failures of electronic boards in the telephone equipment (refer FIG. 3).

L is the inductance caused by conductor loop in the grounding circuit of SPD. Voltage across

L = L.

dI/dT,

where dI is the rate of rise of discharge current through the SPD. PCB fails to this additional voltage impressed between the active circuits of the PCB and its frounding bus.

FIG. 3 Connection of communication equipment

Rectification

The grounding conductor was cut and re-terminated with the shortest possible path for the discharge current flowing from SPD to the ground electrode. There were no further failures.

5 Case study 4

Effect of isolated grounding:

Problem---In a large office complex, computer errors including system crashes/reboots were happening during thunderstorms. The grounding of computer system had been carried out according to the recommendations of manufacturers. The grounding leads were insulated and terminated to an isolated grounding bus. This bus was connected to a grounding electrode consisting of multiple driven rods well away from the building.

Analysis---It was suspected that the isolated computer grounding system was responsible for this problem. Measurements were made using a power quality monitor to confirm this. It was noticed that appreciable voltages were recorded between the building grounding system and the computer ground during electrical storms. This presented a safety hazard to personnel using these computer systems and was also introducing noise into the systems through the capacitances between the computer systems and the building ground (refer FIG. 4).

Note: Lightning current flowing through the lightning conductor to ground electrodes causes the potential of the building members to rise above that of the causing system errors. Unsafe potentials can also arise.

FIG. 4 Isolated grounding

Rectification---The manufacturer of the computer systems was consulted and the computer ground system and the building ground were bonded together on a trial basis. It was noticed that the system problems were no longer occurring. The grounding wires of computer systems were also routed along with the power supply wires to the systems for better performance.

Case study 5

Problem of monitor display:

Problem of wavy monitor display was reported from a new computer installation. Power quality checks did not indicate any abnormality. Subsequently, magnetic field measurements were carried out and indicated presence of high power frequency magnetic fields.

Analysis---When the surrounding area was inspected, it was found that on the other side of the wall where the computer was located, a major power distribution center was located. The high magnetic fields caused by the currents were causing the problem with the monitor display (refer FIG. 5).

FIG. 5 EMI problem

Rectification---The computer system was relocated away from the wall by a distance of about 1.5 m. The waviness disappeared and the display became normal.

Case study 6

Another case of neutral disconnection:

Problem---In an office computer installation, a single-phase distribution board was feeding two 1 kVA UPS systems, each feeding a LAN server. One of the AC units whose power feeder developed a fault was temporarily connected to a spare outlet of the DB feeding the UPS systems. All of a sudden, the circuit breakers on the AC power input of both UPS systems tripped. Power to the AC unit was also interrupted though the feeder to the unit was ON. Switching on the UPS, incoming CBs restored normalcy but the tripping happened again within a few seconds.

Analysis---Voltages were measured between both phase and neutral and between neutral and ground.

The voltages were normal when UPS units were functioning (AC unit was OFF). Currents were measured by a clip-on ammeter. The neutral circuit of the UPS did not indicate any current. When the AC unit was switched on, both neutral circuits indicated higher than normal currents before the circuit breaker interrupted the current.

Further checking showed that the neutral connection at the incoming line to the DB was bad. Also, within the UPS an inadvertent connection between neutral and ground was found (refer FIG. 6a).

Note: Arrows show path taken by neutral current of AC unit due to inadvertent connection between ground and neutral in the UPS unit (see UPS internal connection shown in FIG. 6)

FIG. 6a Distribution arrangement

As long as only UPS units had been connected, the problem with broken neutral connection was not evident since the return current was flowing through the grounding wire of the respective UPS system. When the AC unit was connected, its neutral current took a path through the neutral pole of the circuit breakers of the UPS AC power input (dividing between both UPS breakers). The breakers tripped due to the starting current inrush of the compressor motor of the AC unit (refer FIG. 6b).

Rectification---The neutral connection was restored. The connection between ground and neutral of the UPS systems were removed. The AC unit could be run without any problem till its own supply was repaired. A residual current CB was added in the incoming side of the DB to be able to pinpoint such problems as soon as they occur.

N2 was grounded through G as the Ups output is a separately derived source. Correction N1 N2 is not required but was present however. This correction was responsible for neutral current of AC unit to flow back to the newer source through the grounding wire.

CB tripped due to excessive flow of current in the neutral poles through its over-current release.

FIG. 6b

Current flow after neutral breakage

Case study 7

TV failures during thunderstorm:

Problem---A residential consumer faced a problem of repeated TV set failures whenever a thunderstorm hit the area. The TV had a cable connection. No SPDs were provided either on the power supply wires or the TV cable.

Analysis---It was found that two separate grounds were provided in the house at opposite ends. One was by the power supply company and the other by the cable operator. These were not bonded to each other. The power ground had a connection to the TV through the power cord and the other through the cable screen. Any lightning discharge in the vicinity of the house caused flow of current in the ground, some of which tended to flow through the TV set's components, which provided a parallel path in the absence of a bonding between the two grounds (refer FIG. 7).

Rectification Approved surge protection devices were provided for both power and cable TV circuits.

The ground electrodes were bonded together by an adequately sized copper wire. The problem of TV failure during thunderstorms ceased thereafter.

Case study 8

Potential difference between buildings:

Problem---In an industrial facility, there was a cluster of four buildings each having a process control computer connected with data cabling. Each computer was grounded to the grounding system of the respective building. The grounding systems of the buildings were interconnected through water mains and metallic sheathes of cables, etc. The functioning of the computer systems was very erratic (refer FIG. 8a).

To avoid destructive current flow through TV points A and B need to be connected through a bonding wire.

FIG. 7 Failure of TV set due to multiple (unbounded) grounds

FIG. 8a System as existing

Analysis---It was thought that the possible reason for erratic operation was the flow of stray ground currents through the earthed screens of the data wiring. The grounding of the computer installations of the four buildings was brought together and connected at a single point in one of the central buildings. This immediately caused the erratic operation to cease by eliminating ground loops in the screens of data wiring. However, the new arrangement was a violation of safety codes (refer FIG. 8b).

FIG. 8b System as modified

Rectification---The common grounding point of the computer systems was connected to the grounding system of the building in which the common point was made (which was connected to the grounding system of the other buildings as well). This ensures that no unsafe potentials appear on the enclosures of the computers vis-à-vis the building structures. Another possible method would have been the use of fiber-optic communication cabling (refer FIG. 8c).

FIG. 8c Preferred arrangement.

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